The number of petals a flower has can be an important key to identifying the flower. Newcombe's Wildflower Guide is based on this concept.
An important thing to remember about this concept is that the "number of petals" represents the number of apparent petals. This is not necessarily the botanically-correct number of petals. Oftentimes what appear to be petals are actually sepals or bracts. It is an aid to identification, not a botanical classification.

Asparagus (Asparagus officinalis)
Bellwort, Large-flowered (Uvularia grandiflora)
Bellwort, Perfoliate (Uvularia perfoliata)
Bellwort, Sessile-leaved (Uvularia sessilifolia)
Blue-eyed Grass, Common (Sisyrinchium montanum)
Blue-eyed Grass, Slender (Sisyrinchium mucronatum)
Blue-eyed Grass, Stout (Sisyrinchium angustifolium)
Blue-eyed Grass, White (Sisyrinchium albidum)
Carrion Flower (Smilax herbacea)
Clintonia, Yellow (Clintonia borealis)
Cohosh, Blue (Caulophyllum giganteum)
Cohosh, Blue (Caulophyllum thalictroides)
Colicroot (Aletris farinosa)
Cucumber Root, Indian (Medeola virginiana)
Cucumber, Wild (Echinocystis lobata)
Devil's Bit (Chamaelirium luteum)
Hepatica, Round-lobed (Anemone americana)
Hepatica, Sharp-lobed (Anemone acutiloba)
Hyacinth, Grape (Muscari botryoides)
Leek, Wild (Allium tricoccum)
Lily, Michigan (Lilium michiganense)
Lily, Trout (Erythronium americanum)
Lily, White Trout (Erythronium albidum)
Lily, Wood (Lilium philadelphicum)
Mandarin, Yellow (Prosartes lanuginosa)
Mayapple (Podophyllum peltatum)
Moonseed, Canada (Menispermum canadense)
Onion, Nodding Wild (Allium cernuum)
Solomon's Seal, False (Maianthemum racemosum)
Solomon's Seal, Great (Polygonatum canaliculatum)
Solomon's Seal, Hairy (Polygonatum pubescens)
Solomon's Seal, Star-flowered (Maianthemum stellatum)
Star-of-Bethlehem (Ornithogalum umbellatum)
Stargrass, Yellow (Hypoxis hirsuta)
Tofieldia, Sticky (Triantha glutinosa)
Twisted Stalk, Rose (Streptopus lanceolatus)
Water Shield (Brasenia schreberi)
Yucca (Yucca filamentosa)

Irregular 2 petals 3 petals 4 petals 5 petals 6 petals 7 or more Indistinguishable

THUMBNAILS
Asparagus (Asparagus officinalis)	Bellwort, Large-flowered (Uvularia grandiflora)	Bellwort, Perfoliate (Uvularia perfoliata)	Bellwort, Sessile-leaved (Uvularia sessilifolia)

Blue-eyed Grass, Common (Sisyrinchium montanum)	Blue-eyed Grass, Slender (Sisyrinchium mucronatum)	Blue-eyed Grass, Stout (Sisyrinchium angustifolium)	Blue-eyed Grass, White (Sisyrinchium albidum)

Carrion Flower (Smilax herbacea)	Cohosh, Blue (Caulophyllum giganteum)	Cohosh, Blue (Caulophyllum thalictroides)	Colicroot (Aletris farinosa)

Cucumber Root, Indian (Medeola virginiana)	Cucumber, Wild (Echinocystis lobata)	Devil's Bit (Chamaelirium luteum)	Hepatica, Round-lobed (Anemone americana)

Hepatica, Sharp-lobed (Anemone acutiloba)	Hyacinth, Grape (Muscari botryoides)	Lily, Trout (Erythronium americanum)	Lily, White Trout (Erythronium albidum)

Lily, Wood (Lilium philadelphicum)	Mandarin, Yellow (Prosartes lanuginosa)	Mayapple (Podophyllum peltatum)	Onion, Nodding Wild (Allium cernuum)

Solomon's Seal, False (Maianthemum racemosum)	Solomon's Seal, Hairy (Polygonatum pubescens)	Solomon's Seal, Star-flowered (Maianthemum stellatum)	Star-of-Bethlehem (Ornithogalum umbellatum)

Stargrass, Yellow (Hypoxis hirsuta)	Tofieldia, Sticky (Triantha glutinosa)	Twisted Stalk, Rose (Streptopus lanceolatus)	Yucca (Yucca filamentosa)

Irregular 2 petals 3 petals 4 petals 5 petals 6 petals 7 or more Indistinguishable

To see all Species with 6 Flower Parts in order by Common Name → START HERE